Before commencing excavation or other work where power or other cables may be buried, it is important to determine the location of the cables to ensure that they are not damaged during the work. It is also useful to be able to track the path of buried or otherwise inaccessible power cables. It is known to use detectors that detect the electromagnetic field emitted by power cables carrying alternating currents.
The electromagnetic field emitted by a power cable has a fundamental frequency equal to the frequency of the alternating current carried by the cable. However, harmonic frequencies (multiples in frequency) of this fundamental frequency are generally also emitted. The harmonics are emitted at two, three, four, etc times the fundamental frequency. The second, fourth, sixth etc harmonics are called even harmonics, and the third, fifth, seventh etc harmonics are called odd harmonics. Frequencies that are neither even, odd harmonics nor the fundamental frequency are non-harmonic frequencies.
Cables that do not directly carry currents may also be detected by power currents, as neighbouring power cables, and even overhead power lines can induce signals at power cable frequencies and harmonies thereof onto these cables. Ground return currents from appliances can also travel along non-power cables.
Electromagnetic signals emitted from power and other cables are useful in detecting power and/or other cables because the detector need not be connected to the cable to be located, and the signals are emitted by the cable without any additional reference signal needing to be added to the current flow. In other words, the cable can be in use while it is being detected, and it need not be isolated. Therefore, a passive sensor or detector may be used to detect the cable, and the power consumption of the detector is reduced. However, use of electromagnetic fields in detection can be compromised by high levels of noise being detected along with the signal from the cable to be located. In the present invention, noise relates to spurious, non-periodic noise, and periodic noise outside the frequencies emitted by the object to be located. Such noise problems decrease the accuracy of detection and location, and are therefore undesirable. There is therefore a need to reduce the effects of noise in detected signals, for example in order to more accurately detect/locate buried objects.